Fix wrong sync after UC_ERR_[READ, WRITE, FETCH]_[UNMAPPED, PROT] (#1368)

* Fix wrong sync after UC_ERR_[READ, WRITE, FETCH]_[UNMAPPED, PROT]

Note that:
1. We only guarantee the pc (and other internal states) is correct
if and only of `uc_emu_start` returns without any error (or errors
have been handled in callbacks.).
2. If memory read/write error isn't handled by hooks, the state is
undefined and the pc is probably wrong if no hook is installed.

This fixes #1323.

* Rename variables

* Add note in unicorn.h

* Refine test_i386_invalid_mem_read_in_tb

include/unicorn/unicorn.h

@@ -533,6 +533,10 @@ uc_err uc_mem_read(uc_engine *uc, uint64_t address, void *bytes, size_t size);
         we will emulate the code in infinite time, until the code is finished.
  @count: the number of instructions to be emulated. When this value is 0,
         we will emulate all the code available, until the code is finished.
+  
+ NOTE: The internal states of the engine is guranteed to be correct if and only
+       if uc_emu_start returns without any errors or errors have been handled in
+       the callbacks.
 
  @return UC_ERR_OK on success, or other value on failure (refer to uc_err enum
    for detailed error).